1/18/12 Engineering Drawings

1/11 http-server.carleton.ca/gkardos/88403/drawing/Drawings.htmlWords are not the natural language of engineers.Drawings are their prose, mathematics their grammarand differential equations their poetr.GleggDRAWINGS IN ENGINEERING DESIGNInodcionEngineering drawing is not only the province oI the draItsperson. It is the language oI the engineer. It is theirmeans oI developing and recording their ideas, and conveying them to others. Every engineer will be using andreIerring to some Iorm oI drawings almost daily. They will oIten be producing or directing the preparation oIdrawings. Usually, they make the preliminary sketches and design drawings in accordance with principles oIengineering drawing. Because this is the most unambiguous way oI to convey and record inIormation. It is alsolikely that every engineer at sometime will be checking the work oI designer draIters and approving drawingsbeIore they are sent to manuIacturing. When engineers sign oII the Iinal approval oI a drawing, they takeresponsibility Ior it. An overlooked error in the drawing could be costly.Ideally, then, engineers should be good draItspersons.They can constructively criticize the work oIinexperienced draIters. However, with the limited timeavailable at the University it is not possible to get thenecessary proIiciency. At the university you are giventhe Iundamentals, and it is up to you to improve yourknowledge and skill as required.This course will emphasize design procedures. However, the design drawings which you will be making must beproperly executed.Deelopmen and Podcion DaingIn their everyday work mechanical engineers must be Iamiliar with production drawings. The Iunction oI theproduction drawing is to impart descriptions, speciIications, and instructions to the shop so that three-dimensional objects and systems may be manuIactured and assembled in their correct location with respect toother components oI a machine.Where do the ideas Ior the creation oI the object originate, and how are these ideas developed? The Iorm oI adesign is progressively developed graphically. For example, much oI the original thinking is involved in thetechnical sketch made by the engineer or designer. Many calculations are done at this stage. As IurtherconIirmation oI the practicability oI the design an accurately made scaled drawing called a layout is made. Thelayout shows the overall dimensions and will show several critical elements assembled in their Iunctionalrelationships. Detail drawings are then made. Usually one drawing is made Ior each part, showing completedetails and instructions necessary Ior its manuIacture. Finally, subassembly and assembly drawings are made toshow how the detail parts are to be assembled and to show general dimensions.Specificaion and he Popoal DaingLayout representation begins with the interpretation oI design speciIications by making up proposal drawing (Exhibit 1a , Exhibit 1b , Exhibit 1c ).Suppose a Space Agency wants to purchase a new attitude control system. From preliminary studies they have1/18/12 Engineering Drawings2/11 http-server.carleton.ca/gkardos/88403/drawing/Drawings.htmlOne of he mo efl pocede in all age a hee of feehand keche o epeen all alenaie in a3D aangemen.Nei e al, ICED 83Iound the Ilight characteristics oI their vehicle. This gives them the control requirements Ior their system. A set oIspeciIications is drawn up and requests Ior proposals are issued to the companies Irom which they wish toreceive quotations. The design engineers at these companies on receiving copies oI these speciIications will beginrough designs. The designers will roughly design the components that will make up the system, sensors,actuators, computers, programs, etc. so that a proposal drawing can be made. These drawings show the generaldesign that will best IulIil Iunctional requirements. They show general dimensions, areas, weights and other basicdesign and manuIacturing inIormation. From these proposal drawings a preliminary estimate oI engineering,tooling and production costs are made. The estimated cost and the proposal drawing are sent to the salesdepartment who add a Iactor Ior proIit and establish a selling price which is to be quoted. The price, drawings,and much other descriptive inIormation are then submitted as a proposal and tender.The proposal including the proposal drawings become an essential part oI a design contract, and it is the basis oIthe eventual design, the drawings are not used Ior Iabrication. When a complicated product is being considered,proposal drawings with the written text indicate only the method to be employed, in obtaining basic the Iunctionalrequirements. They emphasize engineering principles to be used in design. The bulk oI the minor design work isgenerally suggested but not completed. It is expected, thereIore, that the Iinal product although constructedaccording to the principles set Iorth in the proposal drawing, may diIIer considerably Irom it.The degree oI completeness oI proposal drawings is inversely proportional to the complexity oI the product.Thus, Ior a less complicated product the proposal drawing may also suIIice as the layout, and occasionally evenas a working drawing. The reason behind this is that involved systems require many specialists who must expendmuch time and eIIort to arrive at detailed solutions. The development procedure is expensive and can be justiIiedonly when an organization has received a contract to carry a design to its completion. Proposal drawings supplyonly enough inIormation Ior contract acceptance.The major product design work begins aIter the company has received the order and the proposal drawing hasbeen accepted. Meanwhile many revisions may have been made to the proposal drawing to suit the customer'srequirements beIore it is accepted. The principal component parts or sections oI the product are assigned tospecialized design groups, and each group might be headed by an engineer. A project engineer will be in chargeoI the complete product design. This however will vary greatly with the type oI organization.The proposal may include an outline drawing ( Exhibit 2 ), at the time oI submission or shortly aIter the order isconIirmed. Outline drawings become part oI the contract obligation. Its purpose is to provide the customerssuIIicient inIormation about the product that they can go on with the rest oI their design. ThereIore the outlinedrawing gives all the dimensions oI the Iinished device requited to attach it and to connect it to the equipment itwill work with. And it must also give the overall dimensions oI the system so that the space it will occupy. Outlinedrawings are sometimes required to be certiIied. A responsible oIIicer oI the company, typically the ChieIEngineer, must sign the drawing guaranteeing that the system will be in accordance with it. Technical SkechThe designers interpret the requirements shown on theproposal drawing, study the accompanyingspeciIications, and begin thinking out solutions. Thesolutions are recorded in technical sketches. In thetechnical sketch ( Exhibit 3a, Exhibit 3b, Exhibit 3c ) the designer puts down the important Iactors - general1/18/12 Engineering Drawings3/11 http-server.carleton.ca/gkardos/88403/drawing/Drawings.htmlshapes, clearances to be checked, structural investigations, Iunctional requirements and basic manuIacturingprocesses that may be used. The designer must exercise ingenuity in making approximations beIore an accuratestress analysis is made to decide actual sizes. Technical sketches are not discarded, they are valuable becausethey record most oI the ideas and the directions that contribute to the Iinal design. As much thinking and planningas possible should be shown in the rough sketches. This expedites a more direct solution and lessens thepossibility oI having to change design principles completely on the careIully drawn layout.The LaoutA layout drawing (Exhibit 4a , Exhibit 4b , Exhibit 4c ) by the designer is an exact graphical representation oI thedesign. It is intended Ior engineering rather than manuIacturing use, although sometimes a layout drawing is usedIor experimental production. The layout is an accurate development oI the conception oI the design, or theplacement oI units. Essential elements are developed, and the geometry oI the machine or structure isdimensionally deIined taking into consideration its Iunction, manuIacture and other requirements. The layout is akey drawing Irom which production drawings are made. Several layouts may be required Ior one machine. Forinstance the steering mechanism in a car would require a layout drawing. The rear end would require another. Inmaking the layout, the basic reIerence lines and center lines are located. Adjacent or existing parts are drawn inphantom lines. This conveniently deIines the space available to work. The general shape oI each componentmember is approximated and calculations are carried out simultaneously which Iinally determines the actual sizes.Sometimes the layout, the design sketches and calculations are made simultaneously because each providesinIormation that is needed Ior the other.Layout drawings are always drawn to scale, Iull scale iI possible. CAD is extremely useIul this way. Layoutdrawing can also be done rapidly on squared paper to give the scale. The prime consideration is accuracy - onlya minimum oI necessary essential graphical inIormation is presented. Layout drawings are similar to assemblydrawings, except that cross hatching is conIined to the boarders and may be done Iree hand. Symbols may beused Ior standard components unless details are required Ior clariIication.For stress calculations Ireehand sketches may be used also. The sketches and calculations are Iiled Ior reIerenceand checking purposes. The coordination oI stress analysis, Iunction, manuIacturing, and clearance Iactors are allembodied in the layout. The Production Detail DrawingDetail drawings ( Exhibit 5 ) represent single elemental components. The drawing contains complete inIormationIor manuIacturing the part.Accepted draIting practice in industries engaged in mass production calls Ior a separate drawing Ior each cast,machined, or Iorged part. These detail drawings are made by detail draIt-persons. They usually obtain the basicinIormation required Ior the part Irom the layout drawing.The person making the layout is usually the engineer or a senior designer. They will be responsible Ior severaldraIt-persons oI diIIerent grades. The detail drawing is critically important because when it is released IormanuIacture it must be a document that has only one interpretation. Once released, the responsibility Ior theaccuracy oI the drawing rests not with the draIt-person, who produced it, but with the designer and/or engineerwho produced the layout and approved the drawing. They thereIore have a critical interest in the production1/18/12 Engineering Drawings4/11 http-server.carleton.ca/gkardos/88403/drawing/Drawings.htmldaig ad hd ae ha i ha bee checed caef befe i i eeaed f aface.Thee ae a ea f eaae a ad dci daig. Seie i a be ibe ace dif a a daig a a bai f a deai daig e a f a aeb daig, b geeahe a daig i ed f deig e . Aeb ad Iaai Ici DaigAfe he deai dci daig hae bee ade, he aeb daig (Ehibi 6 ) i eaed. Thee f he aeb daig i gie a daa eied aebe e a gehe b big,e fiig, edig, ieig e he ce.Aeb daig a a ide ifai f aig a a he ce a. Theaeb daig i eie eeeed a a fia iaai daig, eie hee daig a becaed baebie if he eee he aeb f a ce f a achie hich i ade f eeaa.The aeb daig geea cai he Bi f Maeia. Ahgh, f age ad cicaed achie eachdci deai daig ha i Bi f Maeia. Diii f ab beee Pdc ad T DeigMde deig acice i id ha ce a:dc deig (dc egieeig egieeig) deig ( egieeig deig).Whe de a dci ehd ee begiig, deig ad aface ee cbied. He Fd,f eae, ceaed hi deig ad deeed i ae ad i achie. Egieeig hi dc he ihee ai afacig a feaibe. He he faciiaed deig f he , ehd, ad ceeeied f dci ad a had i he ei aig f aface.A a dci echie aid deeed, igifica diii f ab becae ecea. The fidiii cced beee egieeig deig ad aface. The eed f ecia egieeig i aecgied ad egiee ee aied f ceaie, iiia dc deig. Whe a ccef eeieadeig ha bee deeed, he egiee i eeed ih a eie e e f be cceig he adci f he ie. A hi i a ecd diii f ab cced ihi he egieeig deig a.Mechaica egiee eciaied i afacig -h egieeig. The egiee aed heeibii f aig he dci ehd f he dc ih ii c f ab, aeia ad ie.The a cceed heee ih he ai c f he dc. Tda i id hee i a fhebead ad deig, afacig ehd, ad ai c ae a eaae deae.I he a dci daig ee ceed he e he dci deae f aig ad i he iiiaed he deig. Tda' ceiie eie ha feed he idea f "cce egieeig".I cce egieeig he dc deig, dci, ad ig deig ae bgh gehe ad gehe eeia b cce ha he ig ad dci eiee ifece he dc1/18/12 Engineering Drawings5/11 http-server.carleton.ca/gkardos/88403/drawing/Drawings.htmlFor the designer the purpose of a drawing is two fold.Although it ma eventuall conve information to others,it is first of all an aid to thought.Albert Leerdesign Irom the start. This desirable procedure has been Iacilitated by CAD since several departments can beworking Irom the same set oI data as it is being developed.Tool DeignToo oIten the product engineer does not appreciate the many steps through which a design must go beIore itbecomes an actual interchangeable part. On the other hand the tool engineer is not always sympathetic with manyspecialized problems that conIront the product designer creating the initial design. An understanding andappreciation oI tool problems result in a more eIIicient operation.The tool designers must concern themselves with the Iollowing Iactors:1. Analysis oI the complete manuIacture oI the part;2. Design and manuIacture oI tools and accessories;3. Gauging and inspection oI the Iinished part.Obviously the tool designer must have a thorough knowledge oI machine tools including the various standardsmall tools and accessories. The knowledge oI machine tools and standard small tools must be supplementedwith the ability Ior careIully designing special tools such as jigs, Iixtures, gauges, punches and dies.Although tools Iacilitate mass production, they themselves are custom-made single elements. Tool designprinciples and draIting practices, thereIore, vary Irom production design and drawing techniques. Since toolsusually represent one-oII manuIacture, tool drawings may contain all the detail drawings on the one drawing. Thedrawing may also show in phantom lines or color the outline and location oI the production part it is associatedwith.However, the tool designer still uses the standard stages oI development in drawing a new tool, i.e., the ideasketch, the layout, the production drawing and assembly drawing. Designing With the Layout DrawingGenerally designing takes place in at least two stages: draIt and operational. During the draIt stage the mainarrangement and general design oI a given unit are established (sometimes in several versions). AIter evaluationand discussion oI the draIt, the working operational arrangement is produced, it deIines more accurately thedetails oI the system and serves as the starting point Ior completing the project.During these design stages it is important to identiIy andestablish the principal components, and to Iind thecorrect order oI design and development.Attempting to design the whole system with all itselements at once is a typical error characteristic oI novice designers. Having received the assignment whichpresents the purposes and the perIormance parameters oI the project, the novice designer oIten tries to calculateand complete the design in all its details. then they try to draw all the elements to produce a picture as iI it were aIinished assembly drawing oI the project. Such a procedure is an irrational one, and results in a string oI poorlyarranged constructional elements and units.1/18/12 Engineering Drawings6/11 http-server.carleton.ca/gkardos/88403/drawing/Drawings.htmlIt is preIerable to begin the design with a solution oI principal design constraints, i.e., the selection oI kinematics,the power sources and Ilow, or the correct sizes and shapes oI the main components and oI their most preIerredrelative positions (design sketches). Any attempt to completely describe parts in detail at this stage is not onlyuseless, but harmIul. It draws attention away Irom the main problems oI the design and conIuses the logicaldevelopment the design.Another important procedure Ior design is to Iirst develop several design alternatives concurrently, analyze themand then select oI the best. It is a mistake to set the direction oI the design by accepting the Iirst idea whicharises, or to Iollow an obvious solution. The designer must analyze careIully all Ieasible solutions and choose theone most suitable Ior the given requirements. This requires deliberate eIIort, the problem is not at once solved,but sometimes only aIter long investigation.Full development oI each alternative is not necessary. Usually hand pencil sketches or overlays are suIIicient toestablish the advantages and limitations oI an alternative and to decide whether it is advisable to continue withthat particular alternative.The drawing and the calculations must be carried out in a complementary manner, each contributing to the other.The initial calculations need only be tentative approximations. Main design elements should be evaluated not onlyIor strength, but also Ior rigidity.The designer cannot rely on solely selecting dimensions and shape oI parts by eye. OI course, there are veryskillIul designers who almost without mistakes can establish sizes and cross-sections assuring stress levelsacceptable Ior the given branch oI engineering. Alternately they cannot rely on calculations alone, sketching ordrawing the part to scale can uncover unsatisIactory dimensions and conIigurations. Remember, " II it lookswrong, it probably is wrong". Similarly copying trite shapes and keeping to traditional stress levels, will not createbetter designs.To only depend upon calculations is also wrong. In the Iirst place, the existing methods oI strength calculationsdo not consider many Iactors that inIluencing the suitability oI a design. Secondly, there are some parts andconIigurations (e.g., housings) which cannot be conveniently calculated at all, or the eIIort cannot be justiIied.Thirdly, other Iactors besides strength aIIect the sizes oI parts. For example; the design oI cast parts is dependentlargely on casting technology, parts being machined must resist the cutting Iorces and be suIIiciently rigid, heattreated parts should be large enough to avoid buckling.Thus, besides calculations, the designer must be aware oI existing design practices and regulations and Iollowthem, iI warranted.Another prerequisite Ior good design practice is a continuous consideration oI the manuIacturing problems; Iromthe very beginning ever component should be given a technologically reasonable shape. A skilled designer Iromthe beginning considers how the part will be produced. Novice designers should constantly consult with theproduction and test engineers.The design should be perused on the basis oI standard dimensions (Iitting diameters, sizes oI keyed and splineconnections, diameters oI threads, etc.) where possible. At the same time maximum use oI standard elementsshould be sought. II speciIic elements are necessary in one part oI the system, the same element should be usedelsewhere in the design as much as possible, the objective being to reduce the number oI diIIerent parts.1/18/12 Engineering Drawings7/11 http-server.carleton.ca/gkardos/88403/drawing/Drawings.htmlIn doing the design the designer must take into account all the conditions defining the operate ability of themachine, develop the systems of lubrication and cooling, assembly and disassembly, and attachment of adjacentparts (drive shafts, piping, electric wires, etc.); provide for convenient maintenance, inspection and adjustment ofthe mechanism; choose correct materials for the main components; think of methods for improving the machine'sdurability and wear-resistance of rubbing surfaces, and methods of corrosion protection; investigate anddetermine the limits of the machine when operating under forced conditions.Composition does not always proceed smoothly. Often during designing some small defects, overlooked in thefirst estimates, are revealed. For their elimination it sometimes turns out necessary to return to schemes rejectedearlier or to develop new ones.Some units are not always successfully designed from the first. This should not discourage the designer--theyhave to devise some "tentative" alternatives and raise the design to the required level in the process of furtheractivities. In such cases it is useful, to take a breathing space, after which, as a result of subconscious thought, theproblem is often solved. After a while the designer looks at the outline drawing in another light, and sees themistakes made at the first stage of the development of the main design idea.Sometimes the designers unintentionally lose their objectiveness and do not see the drawbacks of their favoritevariant or the potentialities of other versions. In such cases impartial opinions of outsiders, the advice of seniorsand co-workers should be sought. Their fault-finding and criticism could turn out to be useful. Moreover, thesharper the criticism, the greater is the benefit derived.At all stages of design manufacturers and operators should be consulted.As general rule, the wider the consultation on the design, and the more attention the designer pays to the advice,the better will be the design.The cost of designing is only a small portion of the machine manufacturing expenditures (excluding one-off orsmall-batch production products). In the final analysis, the greater the development work on the design, the moreare the savings in the machine cost, time of manufacture and finishing, the better its quality and the greater theeconomic gains over the machine's service life.If possible the layout is best drawn to a 1:1 scale. This enables a realistic presentation of machine proportions,and facilitates the selection of required dimensions and sections, their strengths and rigidities. In addition fullscale, removes the necessity for a large number of dimensional specifications and simplifies later stages of design,in particular detailing, since dimensions can be taken directly from the full scale layout drawing.Layouts on a reduced scale, particularly less than scale, strongly impedes the design process, it distorts theproportions and reduces the clarity of the representation.If a 1:1 scale is not practical, then at least critical parts and groups should be drawn full scale .The design of simple systems may be developed in one projection if the drawing is sufficiently clear. The cross-sectional drawing can be interpreted in three-dimensions by ones imagination. However, with more sophisticatedsystems, this may cause serious errors; therefore, in such cases the design must be developed in severalprojections for clarity.The development of a lay-out drawing is, a continuous process of search, trial, approximation, seeking1/18/12 Engineering Drawings8/11 http-server.carleton.ca/gkardos/88403/drawing/Drawings.htmlalternatives. Alternatives are compared and the unsuitable rejected. Alternatives should be lightly added to thedrawing and corrected when necessary, which means that an eraser is used more oIten than a pencil.Cross sections can be leIt unhatched, or iI hatched, only Iree hand. Time is not wasted on drawing standard partsin detail. Typical components and units (Iasteners, packing, springs, antiIriction bearings, etc.) should be depictedsimply.Contour outlining, hatching, listing and particulars oI standard small parts are made at the Iinal stage, when thelayout is ready Ior discussion.OIten development drawings are Iree hand, the design drawn with a pencil on a sheet oI squared paper. Suchdrawings have great advantages as to capacity, Ilexibility and easiness oI introducing corrections.This method is especially useIul Ior showing smooth outlines characteristic oI modern designs.The method is convenient Ior designers having certain aptitudes Ior drawing. Some designers are capable, whenapplying this method, oI preparing in a Iew hours complete arrangements, which can be handed over Iordetailing. Drawing Morpholog(From Engineering Graphic Modeling, by E. Tjalve, M.M. Andreasen, F.F. Schmidt)